A. Field of the Invention
The invention relates to a synthesizing method of multi-channel voice signals. In particular, the invention relates to a synthesizing method, which utilizes time-division theory, capable of playing multi-channel voice signals with good voice qualities and easily adjusting the ratio of voice signals.
B. Description of the Prior Art
Conventionally, voice signals can be played via digital/analog conversion (DAC) or pulse width modulation (PWM) in consumer ICs. Synthesis of the multi-channel voice signals can be implemented by current summing or by using adder.
FIG. 1 is a block diagram showing a circuitry for playing four-channel voice signals by using DAC method and current summing. With reference to FIG. 1, the four channels 16 each has an associated voice data generator 11 and an associated D/A converter 12. When the four-channel voice is to be played, each voice data generator 11 generates a voice data that will be converted into an analog signal by the associated D/A converter 12. Then, the output signals of the D/A converters 12 are wire-anded and thus current summing is performed. The summed signal is amplified by a power amplifier 14 and then sent to a speaker 15 to generate the multi-channel voice.
Good voice qualities can be obtained when using the above-mentioned method to play multi-channel voice. However, the disadvantage is that similar circuit unit (DAC circuit) needs to be provided for each channel. Consequently, the entire circuitry will occupy a large area.
FIG. 2 is a block diagram showing a circuitry for playing four-channel voice signals by using an adder. When four-channel voice signals are to be played, the voice data generator 21 of each channel generates a voice data. Then, the two least significant bits of the voice signal in each channel are removed (that is, divide by four), and the voice signal is sent to an adder 22 to obtain a summed data. The summed data are then sent to a D/A converter 23 to be converted into an analog signal. The analog signal is amplified by a power amplifier 24 and then sent to a speaker 25 to generate the multi-channel voice.
The associated disadvantages of the method are that it requires an extra adder circuit and the resolutions of voices are lowered.
FIG. 3 is a block diagram showing a conventional circuitry for playing four-channel voice signals by using a PWM method and current summing. When multi-channel voice is to be played, the voice data generator 31 of each channel generates a respective voice signal. Then, each voice signal is sent to a PWM voice generator 32. Conventionally, the PWM voice generator 32 includes a high-speed counter and a switch. According to the positive phase or negative phase of sound wave, each PWM voice generator 32 converts the associated voice signal into two binary bits, and represents the voice volume by pulse width. Subsequently, the first bit BIT1 and the second bit BIT2 of the output of each channel are wire-anded such that current summing are implemented. The summed signals are sent to a speaker 34 to generate voices.
A disadvantage for playing multi-channel voices using a PWM method and current summing is that similar sound generator circuit unit needs to be provided for each channel. Consequently, the entire circuitry occupies a large area. Moreover, since each output is connected to each other, the output current of each channel flows to other channels, which results in waste of power.
FIG. 4 is a block diagram showing a conventional circuitry for playing four-channel voice signals by using a PWM method and an adder. When the four-channel voice signals are to be played, each voice data generator 41 generates a voice signal. Next, two least significant bits of the voice signal in each channel are removed (that is, divided by four) and then sent to an adder 42 to obtain a summed data. Subsequently, the summed data are sent to a PWM voice generator 43, which includes a high-speed counter and a switch. The output of the PWM voice generators 43 is sent to a speaker 44 for generating voices.
The associated disadvantages of this method is that it requires an extra adder circuit and the resolutions of voices are lowered because synthesis of the four-channel voice signals is based on average pulse width of each channel.
The approaches as described above cannot provide excellent synthesis of multi-channel voice signals. In view of this, a time-division multiplexing (TDM) method, which is widely used in communication systems, is utilized to play multi-channel voice signals, thereby solve the above-mentioned problems.